Generally, in a process of manufacturing vegetable oils such as soybean oil, so-called soapstock is generated. Since the soapstock includes a lot of moisture and oil and is in a form of a highly viscous paste, handling is difficult. Therefore, in the past, most of the soapstock was incinerated as an industrial waste. However, recently an apparatus and a method for producing various kinds of fatty acid and organic fertilizers by using oils contained in the soapstock have been proposed and an effective usage of the soapstock has been tried. For example, Japanese Patent Application Laid-Open No. 2004-91783 (patent document 1) discloses a system for producing a fatty acid ester product and the like by using the soapstock. Also, Japanese Patent Application Laid-Open No. 09-268086 (patent document 2) discloses a method for fertilizing the soapstock.
However, such treatment methods of the soapstock that produces various kinds of fatty acid and organic fertilizers as described above have a problem of treatment efficiency, More specifically, in the soapstock treatment system as shown in the patent documents 1, it takes time for a treatment because there are many processes for treating the soapstock. Also, a maintenance work for operating and controlling each of the processes appropriately becomes complicated. On the other hand, a treatment method of the soapstock described in the patent documents 2, predetermined time is required to ferment and decompose the soapstock. Thus, these treatment methods of the soapstock have a problem that it is not necessarily suitable as a method of treating the soapstock which is produced in great quantities every day.